This invention relates to a technique for permanently fixing the position of objects relative to one another, and specifically to a technique for permanently fixing single-mode optical fibers into position in optical device packages. More specifically, this invention relates to a technique for joining an optical fiber in a high efficiency coupling arrangement with an electrooptical device, such as a semiconductor laser, without the introduction of stresses that invariably result in element misalignment and degradation of coupling efficiency upon subsequent relaxation of such stress.
Alignment of single-mode fibers to optical devices such as lasers or laser amplifiers typically requires alignment tolerances of less than 1 .mu.m for lensed fibers or .+-.2-5 .mu.m if microoptics are used to couple light into the fiber. Regardless of the coupling scheme employed, however, maintaining fiber alignment both during and after the locking process is one of the most difficult, and hence, costly, parts of the packaging assembly process.
Standard locking techniques include epoxying, soldering, and laser welding (see, e.g., M. R. Matthews, B. M. Macdonald, and K. R. Preston, "Optical components--the new challenge in packaging, "IEEE Trans. Comp., Hybrids, Manu. Tech., vol. 13, 1990, pp. 798-806, 1990; and J. Lipson, R. T. Ku, and R. E. Scotti, "Opto-mechanical considerations for laser-fiber coupling and packaging," SPIE Proc., Int. Lens Design Conf., vol. 554, 1985, pp. 308-312). Each of these techniques has disadvantages. Adhesives are inexpensive, but long-term reliability has been a perennial concern both in terms of potential outgassing of organics, and stability over wide temperature ranges, despite recent progress (see, e.g., M. Suzuki, T. Nakanishi, N. Tzuzuki, and N. Murata, "A stable laser module with UV-curable resin for single-mode subscriber use," Proc. 40th ECTC, vol. 1, 1990, pp. 200-205). Solder has the potential to creep, and the high temperatures necessary during the soldering process make active fiber alignment difficult. Laser welding is regarded as having the best long-term stability, but it requires high-power lasers and careful control of a variety of parameters for reproducible results (see Matthews et al, above).
It is also difficult to avoid stress-related misalignments during the fixing process for any of these three techniques. Epoxy has been used most successfully for lens mounting, where only small amounts of epoxy (low strength bonds) are required. If fibers are epoxied, iterative techniques are required to overcome misalignments during the epoxy curing process. Soldering misalignments occur as the solder cools, often requiring rework. Laser welding is highly perturbative, and often requires a post-locking correction, such as bending the fiber tube that holds the fiber.
Electroplating a fiber into position has been found to have several advantages. It requires no special precision matching or expensive pieceparts, and no special equipment. Demonstrations have shown the process to be nonperturbative to at least 1 .mu.m. In U.S. Pat. No. 5,048,919 issued on Sep. 17, 1991 to I. Ladany, coinventor herein, electroplating is shown to fix a single-mode fiber in place while coupled to a laser diode. As described in that patent, the laser is mounted inside a small copper tube, and the fiber, also mounted in a separate copper tube, is butt-coupled to the laser, while the space between the two is filled with an index-matching gel to prevent intrusion of the electrolytic solution during the plating process to the laser and the fiber end. The two copper tubes are then electrically connected by means of a conductive paint, immersed in a plating solution, and plated into a solid copper rod. This results in an extremely small package, with excellent electrical shielding. Disadvantageously, however, the index matching gel used to seal the ends of the two tubes precludes the use of lenses, optical isolators, filters, etc., which might be necessary or useful, depending on the particular application. For example, the higher coupling efficiencies between a laser and a fiber that are necessary for long distance transmission, and that could only be achieved by means of lenses between the laser and the fiber end, could not be achieved.
An object of the present invention is to provide a technique for fixing an object in position relative to some reference base using a low-stress inducing electroplating procedure.
A further object of the present invention is to provide a broad based technique for joining an optical fiber and any general electrooptical device in a high efficiency coupling arrangement using a low-stress inducing electroplating procedure.
A further object is to perform the technique of the invention in a manner that isolates the critical interface between the fiber and the electrooptical device from the plating solution during the electrolytic bonding process in such a way that it is unnecessary for the fiber end and the electrooptical device and its associated components to sit in a protective gel.